The present invention relates generally to methods for calibrating halftone screens, and more particularly, to a method for characterizing a tone response curve for a printing device to enable the calibration of a halftone screen.
Digital halftoning is a process in which digital input signals to a digital printer are modified prior to printing a hard copy, such that a digitally printed version of a photographic image creates the illusion of the continuous tone scale of the photographic original. Most ink-jet printers and electrophotographic xe2x80x9claserxe2x80x9d printers ultimately operate in a binary mode, i.e., a spot is either printed or not at a specified location. Typically, digital halftoning or screening techniques control the printing of spots to obtain the illusion of continuous tones based upon a comparison of the required shade of gray with one of a set of predetermined threshold levels. If the gray is darker than a given threshold level, a spot is printed. If the gray is not as dark as a given threshold level, a spot is not printed.
To print an image using a given halftone screen, the screen must be calibrated for a particular or target printer. Calibration refers to setting the threshold values of the halftone screen (also referred to as generating the screen matrix) so that a given input gray is well represented by the printed image. This calibration is a slow and laborious process that has to be repeated for every halftone screen that one wants to use. The calibration will remain valid as long as the printer characteristics do not change.
The calibration of halftone screens requires the generation of a tone response curve (TRC) for the target printer which determines how much toner will be applied for a given image data input. A conventional process for generating a TRC involves printing test patches for a wide range of dot patterns for a given halftone screen using by the target printer and toner. The gray levels of the printed patches are measured and the measurements are plotted against the actual number of ON pixels in the dot pattern. The plot is then smoothed and the resulting curve is then normalized to form the TRC.
Although the conventional process can be used to generate a good representation of the overall tone response for the printer, it is generally a lengthy and protracted process to print, measure and analyze the large number of dot patterns available in most halftone screens. Additionally, the process is subject to measurement noise, resulting in a TRC that does not accurately reflect the printer""s response. While measurement noise can be reduced by increasing the number of measurements for each given dot pattern, this comes at the expense of increased processing time. Furthermore, the curve smoothing functions employed to remedy the measurement noise can introduce distortions into the TRC.
In accordance with one aspect of the present invention, there is provided a method for characterizing printer response. The method includes (a) generating a plurality of test patches, each test patch comprising a dot pattern having less than three different base patterns for the halftone screen; (b) measuring the luminance of selected ones of the test patches; (c) plotting the luminance measurements to provide a set of calibrated points on a printer calibration curve; and (d) generating the printer calibration curve between adjacent calibrated points using an equation establishing a relationship between ink area coverage and reflectance of the image.